Comets are often found to be active at heliocentric distances far beyond
the limit of AU, within which the activity may be explained by
sublimation of water ice, induced by insolation. The exothermic transi-
tion from amorphous to crystalline ice has long been recognized as a
suitable mechanism for explaining such distant bursts of activity.

Indeed, cometary ice, formed at low temperatures and pressures, should
be amorphous and trap large amounts of gas (CO, CO , etc.), which is
subsequently released during crystallization. Recent observations of
comets indicate a highly porous structure, which is permeable to gas
fluxes. The free gases present in the interior of a comet, due to subli-
mation (in response to internal heating) and release of trapped gas,
gives rise to internal pressures, which may surpass the tensile strength
of the already fragile grainy configuration. This may result in cracking
of the ice matrix and outbursts of gas and dust.

It is shown that the expected effect of crystallization in comets can be
assessed based on simple analytical arguments, involving the time scales
of competing processes. When crystallization of amorphous ice is inclu-
ded in detailed numerical models of the evolution of comet nuclei, it is
found that the process is not continuous, but progresses in spurts.
Their onset, duration and extent in depth are largely determined by the
structure, composition and thermal properties of the nucleus, and, of
course, by the comet's orbit. The results of several calculations that
support and develop this hypothesis are shown (P/Halley, 2060 Chiron,
Hale-Bopp).